The present invention relates to a compressible stop member for use on a crane, such as a fixed mast stop used on a mobile lifting crane, and particularly a fast-acting compressible stop member.
Lift cranes typically include a carbody; ground engaging members elevating the carbody off the ground; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; and a boom pivotally mounted on the rotating bed, with a load hoist line extending there from. For mobile lift cranes, the ground engaging members are moveable ground engaging members. There are different types of moveable ground engaging members, most notably tires for truck mounted cranes, and crawlers. Typically the mobile lift cranes include a counterweight to help balance the crane when the crane lifts a load.
A crane with a pivotable boom will typically include a mast which may be fixed or live. The mast provides a lateral offset from the base of the boom for connection of a crane suspension. A fixed mast will typically include a compressible stop member, often referred to as a mast stop, used to prevent the mast from rotating backwards when there is no load, or a light load on the boom. In normal conditions, the mast stop compresses slowly, but in the event of a sudden loss of weight on the boom, the mast stop must be able to compress quickly. Past mast stops typically included a spring so that the mast stop can engage the mast throughout a range of motion. The spring applies greater and greater force the further the mast stop is compressed. This compression provides a return force to support the mast. A conventional mast stop might be built with a tube inside of a tube, with a spring inside of the tubes. If the mast stop is compressed sufficiently, the spring would be compressed until it reached a solid height, thus greatly increasing the support of the mast stop.
While such mast stops have proven themselves to be adequate, they have a disadvantage in that the spring and tube arrangement cannot be easily scaled up in size due to physical limitations on the space available for the mast stop. For example, a larger crane that has higher capacities may not necessarily be proportionately larger in all dimensions. The larger crane will need a mast stop that can absorb more energy, but the space in which to deploy that mast stop may not be large enough to accommodate the larger spring and tube necessary for the mast stop. Additionally, the typical spring arrangement provides for a limited range of motion over which the mast stop supports the boom and the support that it does provide varies as the mast stop is compressed.
For larger cranes, mast stops have been developed that use hydraulics to provide an extended range of support. Hydraulic valves may be used to control the pressure of the hydraulic fluid in the mast stop. However, the flow rate of a hydraulic valve is directly related to the size of the valve. In order for a hydraulic compressible stop to compress at the same rate as a conventional spring-type compressible stop, the hydraulic compressible stop must use a large valve or multiple smaller valves. Once again, space becomes a problem, along with the added cost and complexity of the valves. Therefore, past mast stops using hydraulic valves may provide an extended range of support, but typically operate slowly, due to the limitations on the valves within the hydraulic compressible stop.
Thus there remains a need for a mast stop that can operate over a larger range of motion, providing a consistent amount of support like a hydraulic mast stop, while being able to be compressed quickly, like a conventional mast stop.